In recent years, there have been demands for higher image quality, longer operating life, and higher speed in image forming apparatuses, such as printers and multi-functional image forming apparatuses. In conjunction with these demands, it has been desired to decrease the particle size of toner for electrostatic development (hereinafter, may be simply referred to as “toner”). However, as the diameter of toner particles decreases, it tends to become difficult to control the charge amount of toner particles. Therefore, there are limits to the extent to which the operating life and speed can be increased.
Under these circumstances, methods are known for controlling the charge amount of toner by changing the type and the amount of charge control agent added which may be constituent of toner base particles or an external additive to be externally added to the toner base particles. As the external additive, hydrophobic silica, titanium oxide, and the like are often used. Recently, resin fine particles have been receiving attention as an external additive.
For example, for the purpose of stabilizing the charge amount of toner even in the case where copying is repeatedly performed for a long period of time or in a special environment, such as a high temperature, high humidity environment or a low temperature, low humidity environment, a developer has been proposed in which positively charged silica fine powder and negatively charged fluorine-containing resin fine particles are used as the external additives to be externally added to toner base particles. Furthermore, in order to prevent the toner from fusing on a photosensitive member and a charging roller which are components of an image forming apparatus and to maintain the charge amount of the developer, a developer has been proposed in which styrene acrylic resin fine particles having two different particle sizes and inorganic fine particles are added to the toner base particles. As in the proposed developers, by using resin fine particles as external additives, the charge amount of the toner can be maintained even after the developer has been used for a long period of time.
However, in resin fine particles which are generally used as external additives, such as fluorine-containing resin fine particles and styrene acrylic resin fine particles which are used in the proposed developers, the charge amount easily increases excessively and the mechanical strength is low. Therefore, the resin fine particles are relatively easily melted, and thus, easily adhere onto the photosensitive member. The toner and the external additives which have adhered onto the photosensitive member are usually removed by a cleaning member or the like for cleaning the surface of the photosensitive member. However, since the resin fine particles have a smaller particle diameter than toner base particles and other external additives (e.g., hydrophobic silica and titanium oxide), the resin fine particles easily slip through the cleaning member and are difficult to remove.
In particular, when printing is continuously performed in a high temperature, high humidity environment, with resin fine particles which have adhered onto the photosensitive member, resin fine particles may further adhere onto the photosensitive member in the shape of a dashed line, such as that formed by scratching with a pen. The portion of the surface of the photosensitive member to which the resin fine particles have adhered is not easily charged. When image formation is performed, since the portion of the surface of the photosensitive member to which the resin fine particles have adhered has a low charge potential, the toner adheres from a developing device to the portion. When the toner is transferred to paper, an undesired black spot or black line appears on the paper image formed. Consequently, when resin fine particles adhere to the surface of the photosensitive member, image defects easily occur.